Flexible media such as paper, plastic, other web or sheet products or the like are commonly imprinted with designs, text or other patterns through the use of ink or some other suitable fluid printing medium. Such printing is often accomplished with flexographic type printing equipment that may be used to spread a desired pattern of the ink or other fluid on the surface of the web or sheet material.
In flexographic printing, the ink or other printing fluid is transferred to the web or sheet material by a plate cylinder having a surface on which a design or pattern is formed. The ink is applied to the plate cylinder by a metering, or anilox roll, which generally has a plurality of relatively small chambers, or cells, distributed over its peripheral surface. The anilox roll may have a surface made of some suitable material, such as a ceramic coating or plating of hardened metal, for example. The cells may be formed in the surface of the anilox roll by engraving, machining or the like.
The peripheries of the anilox roll and plate cylinder contact each other so as to form a nip. This nip, or line of contact, between the anilox roll and plate cylinder permits the ink or other printing fluid to be transferred from the cells of the anilox roll to the plate cylinder.
To ensure high quality printing, it is essential that an even coat of ink first be applied to the anilox roll so that the anilox roll can distribute ink evenly over the surface of the plate cylinder. In the past, various means have been used to distribute ink over the anilox roll. These prior art devices generally fall into two categories, inkwells and fountain rolls. Each of these types of prior art apparatus is described below.
A. Prior Art Inkwells: One prior means uses a chamber having doctor blades positioned in close contact with the surface of the rotating anilox roll. The chamber defines a reservoir of ink that is held against the turning face of the anilox roll. The doctor blades retain the ink within the chamber so as to let the ink pass out of the chamber only when it is carried on the surface of the rotating anilox roll. When this system is used, the ink or other printing fluid is usually introduced into the sealed chamber at a relatively low pressure. One prior art apparatus of this type that has been used to distribute ink on an anilox roll is an apparatus sold by Printco Industries, Ltd. of Wisconsin.
Referring to the drawings, FIG. 1 illustrates a typical prior art apparatus of this type, indicated generally at 10. This prior art apparatus includes an ink well 11 which may contain a quantity of ink or other printing fluid, generally at relatively low pressure. The ink well 11 is in contact with an anilox or metering roll 12. The anilox roll 12 rotates, here shown in a clockwise direction, across the open face of the ink well 11. Thus, a quantity of ink 13 is maintained in contact with a portion of the peripheral surface of the anilox roll 12.
The ink is retained within the well 11 and is metered in a desired quantity on the surface of the anilox roll 12 by doctor blades 14 and 14a. Rotation of the anilox roll 12 carries the ink retained within the cells on the surface thereof past the blade 14 to a nip 15 formed between the anilox roll 12 and a plate cylinder 16. As shown in FIG. 1, the anilox roll 12 and plate cylinder 16 rotate in opposing directions so as to define a point of contact, or nip 15 therebetween. The ink is released from the cells on the surface of the anilox roll 12 and is transferred to the surface of the plate cylinder 16 at the nip 15.
FIG. 1 illustrates that this prior art apparatus has a relatively small enclosed wet cell angle 20 defined by the area of the periphery of the roll 12 that is retained against the ink well 13. In addition, a relatively large area 21 extends between the doctor blade 14 and the nip 15. This area 21 may be known as the empty cell dry angle. Last, a third region 22 of the surface of the anilox roll 12 extends between the nip 15 and the doctor blade 14a. This portion 22 is commonly known as the full cell dry angle.
The flexographic printing equipment utilizing the ink distributing apparatus shown in FIG. 1 is exemplary of many prior art applications in that it has an enclosed wet cell angle 20 of about 25.degree.. The empty cell dry angle 21 may be about 155.degree., and the full cell dry angle 22 may be about 180.degree.. The relative magnitudes of these angles 20, 21 and 22 may vary; however, the foregoing values are representative in that they show that the exposed full and empty cell dry angles 21 and 22 are generally far larger than the enclosed wet cell angle 20.
B. Fountain Rolls: An alternative prior art means for applying ink to a metering roll uses a fountain roll which rotates through an ink reservoir that is spaced apart from the anilox roll. The fountain roll, which is in contact with the anilox roll, is generally made of an absorbent or elastomeric material. Thus, the fountain roll may rotate through the ink reservoir, capture a quantity of ink or other printing fluid on its surface, and carry the ink to the anilox roll. A doctor blade is often located near the nip between the fountain and anilox rolls to remove excess ink or printing fluid that is deposited on the anilox roll by the fountain roll.
A partially schematic environmental view of one of these prior art apparatus 30 is shown in FIG. 2. An anilox roll 37 and a plate cylinder 31 are used, and a fountain roll 32 is used to lift a quantity of ink or other printing fluid 33 from a reservoir 34 to the anilox roll 37. The fountain roll 32 and anilox roll 30 rotate in opposite directions so as to form a nip, or point of constant contact, 35.
Still referring to FIG. 2, the surface of the anilox roll 37 rotates past the nip 35 to a doctor blade 36. The doctor blade 36 is positioned in close proximity to the peripheral surface of the anilox roll 37, often at an acute angle, as shown in FIG. 2. The doctor blade 36 removes excess ink that may have adhered to the peripheral surface of the anilox roll 37 as it passed beyond the nip 35.
Still referring to FIG. 2, the ink on the anilox roll 37 is carried past the doctor blade 36 to a nip 38 formed with the plate cylinder 31. As in the other prior art apparatus, the plate cylinder 31 and anilox roll 37 rotate in opposite directions, such that the ink carried in the cells on the anilox roll 37 contacts the plate cylinder 31 and is transferred thereto at the nip 38 Thereafter, rotation of the roll 37 conveys the emptied cells on the surface of the roll 37 so that they again reach the nip 35 to receive another application of ink from the fountain roll 32.
As shown in FIG. 2, this prior art apparatus has a wet cell angle 40 which extends from the nip 38 to the doctor blade 36. This portion of the rotation is known as the wet cell angle because the excess ink or other printing fluid has not yet been removed by the doctor blade 36. The wet cell angle 40 is exposed, generally, to the atmosphere, and it may vary considerably in size. In the example shown in FIG. 2, the exposed wet cell angle 40 is about 90.degree., or one quarter of the periphery of the roll 37.
An exposed full cell dry angle 41 is defined between the doctor blade 36 and the nip 38. In this example, the full cell dry angle 41 is about 150.degree.. Last, an exposed empty cell dry angle 42 extends between the nip 38 and the nip 35. Again in this example, the empty cell dry angle is about 120.degree.. All of these angles may vary in various prior art apparatus; however, the foregoing values are believed to be representative in that they show that nearly all of the surface of the roll 37 is exposed to atmospheric conditions.
These prior methods have been found useful in many applications. However, as noted previously, good printing quality depends, at least in part, on a uniform distribution of ink on the anilox roll and plate cylinder. For example, if uneven quantities of ink are transferred from the anilox roll to the plate cylinder, streaks or other poor printing results may show in the finished printed product. These problems have been found to be particularly apparent when thin plastic film materials such as polyethylene are printed in flexographic printing equipment.
One phenomenon that has been found to cause poor printing quality, particularly when flexographic apparatus is used to print plastic film, is associated with exposure of the ink or other printing fluid to ambient conditions. Since a relatively large portion of the periphery of the anilox rolls used in prior art printing equipment may be exposed to ambient atmospheric conditions, and because many inks and other printing fluids are relatively volatile, some portion, or all, of the ink or printing fluid may dry or become gummy while in the cells of the anilox roll. As a result, the cells may become clogged over time as the anilox roll 30 in operation. In some instances this drying problem occurs on the portion of the anilox roll surface that is between the doctor blade and the nip between the anilox roll and plate cylinder. Drying may also occur on the anilox roll surface extending between the plate cylinder nip and the fountain roll or inkwell.
The gummed or clogged cells carry less ink to the surface of the plate cylinder and may result in streaks or other poor printing characteristics. In particular, incomplete release of the ink or other printing medium may lead to "ghosting" of the printed medium, a condition which is characterized by the appearance or absence of a certain color or design on the medium being printed at undesired locations. "Ghosting" may occur because the anilox roll and plate cylinder generally do not have the same diameter and therefore turn at different rotational velocities, although the surfaces of the rolls do turn at the same speed so as to maintain a non-sliding point of contact.
For example, if the design surface on the plate cylinder does not contact a particular portion of the anilox roll during a first rotation, the ink in the cells of the anilox roll at that portion is not released to the plate cylinder. Because the anilox roll is exposed to ambient conditions for most of its rotation, the ink in those cells may become dried or gummed, thus occluding the cells. Since the occluded cells cannot pick up ink during their subsequent passes beyond the ink application means, they are unable to transport additional ink to the plate cylinder on subsequent rotations. As a result, the pattern of occluded, or clogged, cells may be transferred to the printed medium.
Other problems are associated with undesired drying of ink or printing fluid in the cells of the anilox roll. For example, gummed or dried ink material may also build up on the leading edge of the doctor blades associated with the anilox roll, which may result in increased wear of the blades and roll.
As a result of the foregoing difficulties, it is often necessary periodically to clean the cells of the anilox roll, thus resulting in down time of the printing equipment which reduces productivity and increases operating costs. Additionally, since gumming of the anilox roll cells may lead to increased doctor blade wear or roll wear, the roll and blades must be removed periodically so that the roll may be replaced or resurfaced and so that the doctor blade may be replaced. Other problems may also require correction from time to time.
The relatively large area of exposure of the ink-coated surface of the anilox roll may also present another difficulty, namely, the fire and health hazard presented by ambient exposure of ink or other printing fluids. The inks or printing fluids that are in common use with modern printing equipment often have volatile, petrochemical based solvents or components. Such materials may be highly flammable, and they may also be carcinogenic or otherwise hazardous to the health of persons who work with the printing equipment or in areas adjacent thereto. An additional problem may result from air becoming entrapped in the cells of the anilox roll thus blocking a new supply of ink from being delivered to the cells. This problem may be exacerbated in ceramic coated rolls having relatively slick surfaces. Thus, it is essential to force the air out of the cells to permit new ink to be fed to the cells for transport to the plate cylinder.
Similar problems to those described hereinabove may also occur in gravure and other types of printing equipment.
One prior art patent that shows an inking unit is U.S. Pat. No. 4,998,474, issued to Hauer on Mar. 12, 1991. The Hauer '474 patent shows a plurality of ink wells and doctor blades. However, the apparatus shown in this patent does not significantly reduce the portion of the anilox roll that carries ink that is exposed to atmospheric conditions. Another prior apparatus is shown in U.S. Pat. No. 2,151,969, issued to C.L. Henderson on Mar. 28, 1939. However, this apparatus likewise does not show a significant covering of the surface of the print roll, and it is not intended for use with anilox rolls of the type used in flexographic printing but is instead used for rotogravure type equipment. Another reservoir for applying ink to a roll is shown in U.S. Pat. No. 4,009,657, issued to Bonanno et al. on Mar. 1, 1977. However, none of these prior apparatus are believed to have been satisfactory in solving the problems described herein.
In light of the aforementioned deficiencies, it is an object of the present invention to provide a new ink distributing apparatus for use with anilox or gravure rolls which provides improved printing characteristics.
Another object of the present invention is to provide an ink distributing apparatus which provides for even distribution of ink on a metering roll such as an anilox or gravure roll.
Still another object of the present invention is to provide an ink distributing apparatus for use with printing equipment which reduces the exposed area of the full and dry cells of an anilox roll to reduce gumming or clogging of the cells in the anilox roll.
A further object of the present invention is to provide an ink distributing apparatus which forces undesired air from the cells of the metering roll to permit a new supply of ink to be delivered to the cells.
Yet another object of the present invention is to provide an ink distributing apparatus which reduces doctor blade wear.
A still further object of the present invention is to provide an anilox roll ink distributing apparatus which reduces the down time necessary for replacing, repairing or cleaning anilox rolls and doctor blades.